Why read old books?

There is a strange idea abroad that in every subject the ancient books should be read only by the professionals, and that the amateur should content himself with the modern books....This mistaken preference for the modern books and this shyness of the old ones is nowhere more rampant than in theology....But if [a beginner] must read only the new or only the old, I would advise him to read the old. And I would give him this advice precisely because he is an amateur and therefore much less protected than the expert against the dangers of an exclusive contemporary diet. A new book is still on its trial and the amateur is not in a position to judge it. It has to be tested against the great body of Christian thought down the ages, and all its hidden implications (often unsuspected by the author himself) have to be brought to light....
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books....We may be sure that the characteristic blindness of the 20th century...the blindness about which posterity will ask, 'But how could they have thought that?'...lies where we have never suspected it .... None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books. The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books. Not, of course, that there is any magic about the past, ... People were no cleverer then than they are now, they made as many mistakes as we. But not the same mistakes.

C S Lewis, "Introduction to Athanasius' On the Incarnation" (1946)